I may have this all wrong- but in looking at spring rates (rear) the Z28 shows a rate of 103, while other base models have up to 126 Lbs, while the 427 CI option shows a 4 leaf 126 Lb spring. I used the MOOG 6308 in my Z28 as a replacement, and additionally Mr. Pozzi recommends 125-150 rear rates- so why did GM choose to use such a "light" spring on the "HD" model(s), which would seem to make the car prone to oversteer?